While pH does affect Cation Exchange Capacity I do not think it will extinguished at any pH there is a sweet spot however general pH lower and higher than neutral can lower CEC.

Attach look at how EC is changing with pH be never becomes extinct in the normal pH found in a natural condition.

Dear Xavier Dupla

Have a look at this published paper. hope be useful.

http://landresources.montana.edu/nm/documents/NM8.pdf

Soil pH is highly important for CEC as at less acidic pH, no of negative charge on colloid increases, thereby increasing CEC. I agree with Dr. Paul that CEC is zero at any pH

Thank you all for your answers. Sunita Gaind do you confirm that the n° of negative charges increases with pH because functional groups (mainly alcohol and carboxylic acid) get deprotonated?

The organic matter CEC= 0, when the pH of the solution, achieve pH= 2,5. This is the Zero Charge Point, where all the functionnal groups are protonated.

Thank you very much for this helpful answer # Roberto Antunes Fioretto . Would you have a reference to recommend on the subject?

Thanks a lot in advance

Personal information.

Ok thank you !

oerganic matter is so variable in nature and composition and the chemistry of the water or soil varies that there are too many factors to consider to dtermine a CEC of zero.

However generally organic rich waters have a pH of 5-6 buffer range while carbonic acid is closer to 6.

Following .

regards

Hi all,

Here is what I found in Environmental Soil Chemistry from Donald Sparks (2003) :

"Organic matter is a variable charge soil component. Since its point of zero charge (PZC), defined as the pH at which the colloid particle has no net charge, is low, about 3, SOM is negatively charged at pH values greater than 3. As pH increases, the degree of negative charge increases due to the deprotonation or dissociation of H+ from functional groups."

Xavier, about your citation from Sparks (2003):

When pH = PCZ, the solid phase (i. e. organic matter) still contains both, negative and positive charges; However, the **net** charge is zero, which means that the negative charges minus the positive charges = 0. So, when pH = PZC, you will still have both, CEC and AEC (anion exchange capacity).

I am not sure if there is a pH value (within the 0-14 range) in which the CEC of organic matter is zero, but if there is, it is below the PZC (the PZC of organic matter is commonly between 2-3, as you mentioned).

I hope I've helped.

Thank you very much Thadeu for your answer. I did not know that OM could have positive charges (and therefore an anion exchange capacity) at a certain pH. Could you tell me more about how this is possible?

Thank you again alot,

Xavier

Sure, Xavier.

Organic matter has many charged radical groups. Depending on the polarization of these groups, they will have a higher or lower capacity to bound with H+. This polarization depends mostly on the electronegativity of the atoms constituting these radicals. Some radicals, such as amino groups have pKa > 8.00 (in general, N-related groups present high pKa). This means that, at this pH value, half of the amino groups will be as NH2 and half will be as NH3+. As pH drops, the proportion of NH3+ increases. As they are positively charged, they can attract anions.

I am pretty sure I already saw some material talking about this, but I couldn`t find it now. What you can find very easily is the behavior of amino-acids functional groups. Despite the possibility of some differences on the pKa values, the general mechanisms of protonation should not be different. I`ll let you know if I find something more specific. You can also take a look at some general colloidal chemistry book.

Thank you immensely Thadeu for this precious information !!

at the isoelectric point